Piezoelectric resonator circuit



Feb. 24, 1942. w O 2,274,486

PIE'ZOELECTRIC RESONATOR CIRCUIT Filed May 18, 1940 -mmmtot Patented Feb. 24, 1942 UNITED "sTa'rlazs PATENT OFFICE.-

l PIEZOELEC TRIC RESONATOR CIRCUIT Winfieldlt. Koch, Haddonfleld, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application May 18, 1940, Serial No. 335,963

Claims. (Cl. 179171.5)

This invention relates to piezo-electric resonator circuits adaptedgenerally for filtering, oscillation generation, and frequency modulation.

It is an object of' this invention to provide an improved circuit and means for changing the resonant frequency and the damping of piezoelectric resonators in selective signal transmission, oscillation generating and frequency modulation circuits, without changing the constants or arrangements of such circuits, as has been necessary heretofore, with known circuits of the character referred to.

It is also an object of this invention to provide an improved piezo-electric resonator system and control circuit therefor which is readily adapted for the generation and control of frequency modulated signals.

In accordance with the invention, the coating on the crystal surface or the electrode area of a crystal element in a signal conveying circuit is divided, and a segregated area or certain segregated areas are connected to a suitable reactance or resistance network included in a separate control circuit by which the frequency and ,damping of the signal conveying circuit through the crystal or piezo-electric resonator is controlled, the control circuit being coupled with the signal channel only through the crystal element of the resonator circuit.

The invention will, however, be better understood from the following description when .considered inconnection with the accompanying drawing, and itsscope is-pointed out in the appended claims.

In the drawing,

Figure 1 is a schematic circuit diagram of a piezo-electric resonator circuit embodying the invention, and

Figure 2 is a further schematic circuit diagram of a modification of the circuit of Fig. 1, also embodying the invention and adapted for frequency modulation.

Referring to Fig. 1, a portion of a high frequency signal transmission circuit is shown, comprising input terminals 5 and output terminals 6 between, which are connected two amplifier stages in cascade, comprising amplifier {tubes land8. Y

The first stage amplifier- 1' comprises an input grid circuit 5 having an input impedance l0 connected. across the terminalsj, and an output anode circuit ,l I which; includes the tuned primarywinding l2of ahigh frequency-interstage coup i strans rmen. 1 .T a seqqn em t itthe' ran o m s.

nected through a balanced crystal circuit I5 with the grid input circuit l6 of the second sta e 8 which includes a tuned circuit l1, and the output anode circuit I8 is connected with the terminals li The tuned circuits shown may be tuned to the crystal frequency, in which case the circuit operates to convey signals at that frequency, amplifled in thestages I and 8, from the input terminals 5 to the output terminals 6.

The crystal circuit is provided with a grounded center tap -20 on the secondary l4 and a circuit from one terminal of the secondary to an input electrode 2| on the crystal element 22. An output electrode 23 is connected to the high side of the input circuit l6 and through a neutralizing capacitor 24 to the opposite terminal of the secondary. Signals are conveyed through the circuit between the ground connection 20 and the high side of the circuit, thence through the crystal to the imput circuit IS. The crystal capacity is neutralized by proper adjustment of the capacitor 24. The crystal may be of any suitable material, such as Rochelle salt or quartz.

A suitable area of the crystal, segregated and apart from the areas occupied by the main electrodes 2| and 23, is provided for signal control purposes, in accordance with the invention, with auxiliary control electrodes. In the present example, two such separate electrodal areas are indicated at 26 and 21 at the opposite end of the crystal from the electrodes 2| and 23. The control or auxiliary electrodes are connected with a control circuit 28 which includes a predetermined resistance or reactance 29, together with a tuned circuit 30 in shunt therewith and effective to tune out the inherent shunting capacity of the electrodes 26 and 21 across the impedance or reactance device 29.

The reactance device is preferably a resistor, as shown, having no appreciable frequency characteristic and the circuit 30 preferably includes a tuning inductance 3| and adjustable shunt capacitor 32, this circuit being preferably tunable through a frequency range determined by the-desired range of ,variation of the frequency of the resonator circuit.

I With the circuitarrangement shown, the frequency and damping of the resonator circuit l5 may be changed without change in the associat'ed input and output circuits. as commonly required, and without changing the dimensions of the resonator circuit electrodes.

Bydividing the coating or electrode area on the crystal surface, as shown, and connecting a suitable reactance or resistance to the segregated area, it has been found that changes in frequency or damping may readily be obtained as a result of varying the reactance or resistance in the control circuit, or the resonant frequency of the resistive component of the crystal impedance for maximum damping and attenuation of signals through the crystal,

For increasing the damping, a resistance device such as that shown at 29 may be made to match that of the segregated area between the electrodes '26 and 21 so that the resistance device will absorb power sufflcient to reduce effectively the selectivity afforded by the areas 2| and 23 connected with the signal conveying circuit.

Furthermore, by providing segregated electrode or coating areas on the crystal, the control circuit may be operated at low potential to ground. Thus, as shown, one of the electrodes 21 may be connected to ground along with the low potential side of the impedance 29 and the tuned circuit 30, as indicated at 35. With this arrangement, the electrodes 26 and 21 and the control circuit have no appreciable undesired effects upon the signal selecting portion of the system, such as the crystal resonator circuit 15.

The impedance of the element 29 is preferably higher than the effective impedance of the parallel resonant circuit 30 and may be of the order of several thousand ohms, or may be increased above that value with regeneration applied thereto.

With the circuit 30 tuned to resonance with the crystal, variation of the impedance 29 causes the impedance of the crystal in the signal conveying resonator circuit I5 to vary, thereby causing the signal transmission or gain through the circuit to vary, together with the selectivity of the circuit I5.

It has been found that by varying the tuning of the circuit 30 from resonance, that is, by detuning the circuit 30, the frequency response of the crystal may be varied in a relatively wide range on either side of the resonance frequency. Therefore, this arrangement provides means for shifting the frequency of the piezo-electric resonator circuit without direct connection with the circuit and wholly through the crystal coupling. Variation of the impedance 29, with the control circuit 30 detuned, causes a change in the phase and amplitude of signals transmitted through the circuit l5, and therefore variation of the impedance under such conditions provides ready means for phase and amplitude modulation of signals transmitted through the piezoelectric resonator circuit.

Referring now to Fig. 2 in which the same ref erence numerals are used to designate like circuits and circuit elements, the amplifier stages 1 and 8 which are connected between the input terminals 5 and the output terminals 6 as in the circuit of Fig. l. The interstage coupling system is a modification of the piezo-electric resonator circuit of Fi 1 in which the crystal is indicated at 40 having on one face thereof a grounded electrode or coating 4| and on other or opposite faces a plurality of electrode or coated areas indicated at 42. 43 and 44, the first and-the last being the cr stal input and output electrodes, respectively. The input electrode 42 is coupled through a capacitor 45 with the output circuit ll of the first stage 1 across a suitable anode impedance, preferably an anode re sistor 46 having no appreciable frequency char- 5 acteristic. The output electrode 44 is connected with the input circuit [6 of the output stage 8' across a grid resistor 41.

With this arrangement, signals areconveyed through the crystal coupling between the elec- 10 trodes 42 and 44, from the output circuit H to the input circuit IS. The control circuit in the present example is connected between the segregated electrodal area provided at 43, preferably intermediate between the electrodes 42 and 44,

5 and ground or the opposite common electrode 4!,

The control circuit includes a lead 50 connected between the electrode 43 and a coupling erably of relative low impedance or reactance at the resonant frequency or circuit 53.

As in the circuit of Fig. 1, variation of the tuning of the control circuit 53 from resonance, that is, from the crystal frequency, is effective to cause a shift in the resonant frequency of the crystal 4B and this may be controlled by any suitable means. For example, the variation in frequency may be made in accordance with an audio modulation frequency by connecting a variable reactance tube 60 with the circuit 53, as shown, the plate 6| of the tube being connected through the inductance 54 to a positive supply source indicated by the lead 62 and being coupled to the control grid 63 through .a coupling ,or feedback capacitor 64.

,6 In the present example, the tube 60 is of the screen grid pentode type having a cathode 65 connected to ground through a suitable bias resistor 66 provided with a bypass capacitor 61. A variable bias is derived from the resistor 66 through a tap connection 68 connected back to the control grid 53 through an input circuit comprising a transformer secondary 69 and a grid resistor 10. By varying the bias potential provided by the tap 68, the loading of the tube 60 may be controlled and adjusted to such a value that the circuit 53 is in resonance with the crystal frequency, or the adjustment may provide for shifting the resonant frequency of the crystal as desired.

The resonant frequency of the circuit 53 may then be further shifted to vary the resonant frequency of the crystal circuit by applying modulation signals to the grid 63 through the transformer 69 from any suitable source such as a microphone H.

In order to produce frequency modulation, oscillations representing the mean frequency or carrier are transmitted throu'ghthe crystal circuit, for example, by connecting the high potential output terminal 6 through a feedback circuit 70 13 to a feedback input terminal 14 which is connected to the high potential side of the input circuit 5. thereby causing the system to oscillate at the crystal frequency. The strength of'oscillations is determined by a series controlling resistor 16 in the feedback circuit. and the output-anode circuit. I8 is decoupled for. direct currents, from the input circuit, .by a capacitor I1 also in the lead 13..

It will be noted that the input electrode 43 and the output electrode 44 are shielded from the control electrode 43 on the crystal by shield plates 18 interposed between the electrodes and connected to ground 80. This effectively prevents any interaction between-electrodes except through the crystal. i

From the foregoing description, it will be seen 4 that a piezo-electric resonator in a signal-conthe piezo-electric resonator may be varied on either side of the resonance by means of suitable impedance crreactance devices in the auxiliary control circuit, and auxiliary electrode means associated with the piezo-electric device in the resonator circuit associated with areas of the said device not associated withthe main electrodes.

In any system embodying the invention, it will be seen that the piezo-electric device in the resonator circuit is provided with signal input and output electrodes and an auxiliary electrode preferably shielded from the input and output electrodes and connected with the isolated control circuit containing the tuning or impedance varying device or circuit network.

Amplitude and phase modulation results from detuning the control circuit from the crystal frequency and varying the impedance of the circuit, whereas amplitude control alone is provided when the control circuit is tuned to th crystal frequency.

Frequency modulation of oscillations transmitted through the crystal or piezo-electric device may be provided in the same manner through the use of the auxiliary electrode connection with the crystal and the auxiliary control circuit containing a variable reactance device responsive to the modulation signals. It is apparent that the reactance device 60 may be replaced by any I and means for adjusting the resistive component of said impedance to a value of the order of the value of the resistive component of the crystal 7 impedance.

2. In a high frequency signal conveying system, a piezo-electric crystal-controlled oscillation-generating circuit, oscillation control means including a piezo-electric crystal and a pair of signal conveying electrodes therefor, circuit means connected with said electrodes for conveying signals through said crystal as a circuit coupling element, a third electrode for said crystal in spaced relation to said first-named electrodes, and a variable reactance circuit connected with said third electrode for. varying the resonant frequency of the crystal.

3. In a high frequency signal conveying system, a piezo-electric crystal-controlled oscillation-generating circuit, oscillation control means including a piezo-electric crystal and a pair of signal conveying electrodes therefor, circuit means connected with said electrodes for conveying signals through said crystal as a circuit coupling element, a third electrode for said crystal in spaced relation to said first named electrodes, a variable reactance circuit connected with said third electrode for varying the resonant frequency of the crystal, and means providing a shield between said third electrode and said first named electrodes.

4. In a high frequency signal-conveying system, a piezo-electric resonator circuit comprising a piezo-electric crystal for determining the frequency of signals transmitted therethrough, means providing a signal input and a signal output electrode for said crystal, signalconveying circuits coupled to said input and output electrodes responsive to signals at a frequency of the order of the crystal frequency, means providing an auxiliary electrode associated with said crystal, and means providing a reactance in circuit with said auxiliary electrode for determining the frequency of oscillation of said crystal between predetermined limits.

5. In a high frequency signal-conveying system, a piezo-electric resonator circuit comprising a piezq-electric crystal for determining the frequency of signals transmitted therethrough, means providing a signal input and a signal output electrode for said crystal, signal conveying circuits coupled to said input and output electrodes responsive to signals ataa frequency of the order of the crystal frequency, means providing an auxiliary electrode associated with said crystal, means providing reactance in circuit with said auxiliary electrode for determining the frequency of oscillation of said crystal between pre-- nected with said electrodes for conveying oscillations through said crystal device as a circuit coupling element, an auxiliary control electrode associated with said crystal device, an auxiliary control circuit connected with said lastdnamed electrode, variable impedance meansiiii' said circuit, and means in shunt therewith for neutralizing the capacity of said electrode.

7. In a piezo-electric crystal-controlled signalconveying system having a piezo-electric crystal provided with signal input and output electrodes, signal-conveying circuits connected with said electrodes, circuit means connected with said electrodes for conveying signals through said crystal as a circuit coupling element, an auxiliary electrode associated with said crystal in spaced relation to'said first-named electrodes, a tuned circuit connected with said last-named electrode, and means for varying the reactance of said circuit thereby to vary the signal transmission characteristic of said crystal.

8. A piezo-electric resonator circuit comprising, in combination, a piezo-electric crystal device having an input electrode and an output electrode, means connected with said electrodes for conveying electrical oscillations through said crystal at a frequency of the order of the crystal frequency, an auxiliary electrode for said crystal, a variable reactance circuit connected with said electrode, and means responsive to sound frequencies for varying 'the reactance of said lastnamed circuit.

9. In a high frequency signal-conveying system, the combination with a signal amplifier, of an interstage coupling network comprising, in combination, a piezo-electric crystal having an input electrode and an output electrode, circuit means connected with said electrodes for conveying signals through said crystal as a circuit coupling element, a pair of additional electrodes associated with a predetermined area of said crystal in spaced relation to said first-named electrodes, a control circuit connected between said last-named electrodes, and means providing a variable impedance in said control circuit tor .varying the transmission characteristic of said first-named circuit means.

10. In a high frequency signal-conveying system, the combination with a signal amplifier, of an interstage coupling network comprising, in combination, a piezo-electric crystal having an input electrode and an output electrode,,circuit means connected---with said electrodes for conveying signals through said crystal as a circuit coupling element, a pair of additional electrodes associated with a predetermined area of said crystal in spaced relation to said first-named electrodes, a control circuit connected between said lastnamed electrodes, means providing a variable impedance in said control circuit for varying the transmission characteristic of said first-named circuit, one of said last-named electrodes being grounded, and means providing a grounded screen between at least one of said auxiliary electrodes and said first-named electrodes.

WINFIELD R. KOCH. 

